1. Field of the Invention
The present invention generally relates to biosensors and methods for making biosensors. The present invention also relates in particular to electrochemical biosensors for sensing blood glucose levels and methods for making electrochemical biosensors for sensing blood glucose levels.
2. Discussion of the Background Art
Many people can benefit from conveniently and reliably monitoring one or more physiological parameters. For example, diabetics must generally monitor their blood glucose level on a frequent basis and may do so using various diagnostic systems. These systems typically include a test strip and a meter configured to determine the blood glucose level in a blood sample introduced to the test strip. More generally, it can be very useful in many industrial settings to conveniently and reliably monitor various samples (e.g., food, water, oil, chemicals, etc.) for various types of constituents, whether to detect the presence of normally absent constituents, to ascertain the concentration of certain constituents, etc.
The aforementioned test strip and meter may determine the blood glucose level (or another relevant physiological parameter) in a blood sample using various technologies. Among these, electrochemical technologies are prevalent because they allow the relatively rapid and accurate determination of the blood glucose level using a very small blood sample, usually less than 1 μl.
An electrochemical test strip contains a biosensor typically formed in a sample cavity containing suitable reagents and electrodes. In the case of diabetics seeking to measure their blood glucose level, reagents suitable for glucose will, upon introducing a blood sample into the sample cavity, react with any glucose present in the blood sample. Thereafter, in amperometric systems, the meter will apply a voltage to the electrodes to cause a redox reaction, resulting in a current from which the blood glucose level may be determined. Such exemplary biosensor systems can be found in commonly-assigned U.S. Pat. No. 6,743,635, which is incorporated herein by reference in its entirety. Other methodologies, such as potentiometry and coulometry are also known in the art.
There is an ongoing need for test strips and meters that are accurate and reliable because a precise knowledge of blood glucose levels may be critical to the health of a diabetic. Moreover, evolving market demands are imposing new and increasingly stringent performance requirements, including the ability to measure blood glucose levels using smaller blood sample sizes and the ability to complete those measurements in a shorter time. These new requirements entail reductions in the dimensions of the test strip and its sample cavity and electrodes, and, in turn, create new manufacturing challenges.
Among these new manufacturing challenges are those involving the uniform introduction of a small sample to the biosensor. The biosensor's reagents can be disposed on any surface of the sample cavity, e.g., using micropipetting or aerosolization, or any other technique known in the art to deposit fluid onto a small surface. When the sample enters the sample cavity and comes into contact with the surface covered by the reagent, the reagent can react with the target analyte in the sample. When the volume of the reagent is relatively large, uniform spreading occurs relatively easily. However, when the volume of the reagent is small, the reagent will usually be disposed on the surface in small droplets that generally tend to either remain at the location where they were dispensed, without uniformly spreading, or, at best, spread very slowly. Surfactants may be added to the reagent to lower its liquid surface tension and thereby facilitate spreading to an extent, but the use of surfactants does not sufficiently accelerate spreading and often fails to promote spreading sufficiently to fully cover the surface of the sample cavity designed to be exposed to the sample.
In addition, the slow spreading of the reagent on the surface of the sample cavity can be a significant problem because it leads to manufacturing delays, since spreading must be complete prior to drying the reagent, and because such slow spreading causes non-uniformity of the reagent layer, which can negatively impact test accuracy and precision.
“Ultrasonic sprays,” where a solution to be sprayed or deposited is atomized using an ultrasonic nozzle before being sprayed, have been used to create uniform depositions, See U.S. Pat. Nos. 5,451,260; 6,468,605; 6,583,071; and 6,706,337, which are all incorporated herein in their entirety. Ultrasonic vibrations have also been used in other contexts, such as ion bombardment or radioisotope coating. See U.S. Pat. Nos. 6,835,523; 5,932,302; and 6,676,988, which are all incorporated herein in their entirety. Finally, ultrasonic vibrations have also been used to coat a dipped optical fiber. See U.S. Pat. No. 4,639,078, which is incorporated herein in its entirety. However, none of these references pertains to the spreading of droplets or addresses the aforementioned biosensor manufacturing challenges.